1. Field of the Invention
This invention generally relates to classifiers used for separating fine particles from a particulate material. More particularly, this invention is directed to a centrifugal gas classifier equipped with a rotating member that improves particle separation and finer cutpoints, such as particle sizes below fifteen micrometers.
2. Description of the Prior Art
Classification is a process for separating particles below a targeted diameter, referred to as the cutpoint, from a powder or other particulate material. Various types of gas (air) classifiers are known for classifying particulate materials, such as construction materials and fine powders used in toners of copiers and printers. Classifiers can be categorized as either dynamic classifiers with rotating parts such as discs, fans and turbines, or static classifiers without moving parts. The latter includes gravitational, gravitational-inertial and centrifugal classifiers, examples of which include U.S. Pat. Nos. 3,240,335 and 3,397,780. Static classifiers provide the advantages of high efficiencies with lower power requirements and lower operating costs as compared to dynamic classifiers.
The type of static classifier used is typically dependent in part on the capacity, feed rate, and cutpoint control required for the particular operation. For example, gravitational classifiers are suitable for cutpoints on the order of about ten to one hundred mesh (about 1650 to about 150 micrometers), while gravitational-inertial classifiers utilize increased air velocities to achieve lower cutpoints, e.g., about fifty to two hundred mesh (about 300 to about 75 micrometers). Cutpoints as low as about fifteen micrometers can typically be achieved with centrifugal classifiers, which employ an air stream flowing in a circular path. Internal baffles apply a drag force to coarser particles of the particulate material, while the circulating air is directed through the material to separate the finer particles. Particles above the cutpoint eventually drop to the bottom of the classifier where they are discharged, while the finer particles continue on a spiraling path toward a central outlet.
While cutpoints above fifteen micrometers are appropriate for many applications, powder materials for certain applications, including copier and printer toners and pigments for various uses, must often be finer. For this reason, dynamic classifiers are often used in these applications. Because conventional dynamic classifiers have significantly higher power requirements and operating costs than static classifiers, it would be desirable if a lower cost alternative was available to serve the toner and pigment markets, as well as other industries that require particulate materials with particle sizes of less than fifteen micrometers.